Deep-Sea Research II 46 (1999) 1137}1168
An inverse modeling study in Fram Strait. Part II:
water mass distribution and transports
Pawel Schlichtholz , Marie-NoeK lle Houssais*
Institute of Oceanology, Polish Academy of Sciences, Sopot, Poland
Laboratoire d'Oce& anographie Dynamique et de Climatologie, UMR CNRS/ORSTOM/
Universite& Pierre et Marie Curie, Paris, France
Received 10 March 1998; received in revised form 15 November 1998; accepted 25 November 1998
Abstract
A water-mass analysis is carried out in Fram Strait, between 77.15 and 81.153N, based on
three-dimensional large-scale potential temperature and salinity distributions reconstructed
from the MIZEX 84 hydrographic data collected in summer 1984. Combining these distributions with the geostrophic #ow "eld derived from the same data in a companion paper
(Schlichtholz and Houssais, 1999), the heat, fresh water and volume transports are estimated for
each of the water masses identi"ed in the strait. Twelve water masses are selected based on their
di!erent origins. Among them, the Polar Water (PW) enters Fram Strait from the Arctic Ocean
both over the Greenland Slope and over the western slope of the Yermak Plateau. In the
Atlantic Water (AW) range, four modes with distinct geographical distributions are indenti"ed.
In the Deep Water range, the Eurasian Basin Deep Water (EBDW) is con"ned to the Lena
Trough and to the Molloy Deep area where it is involved in a cyclonic circulation. The warm
and shallower mode of the Norwegian Sea Deep Water (NSDW), concentrated to the west, is
mainly seen as an out#ow from the Arctic Ocean while the cold and deeper mode, essentially
observed to the east, enters the strait from the Greenland Sea. Apart from the EBDW, there is
a tendency for all water masses of polar origin to #ow along the Greenland Slope. The two most
abundant water masses, the AW and the NSDW, occupy as much as 67% of the total water
volume. The southward net transport of PW through Fram Strait is about 1 Sv at 78.93N. At
the same latitude, the net transport of AW is southward and equal to about 1.7 Sv. Only the
transport of the warm mode (AWw) is northward, amounting to 0.2 Sv. The overall net out#ow
of the Deep Waters to the Greenland Sea is about 2.6 Sv. Two upper water masses, the fresh
(AWF) and the cold (AWc) mode of the AW, and one deep-water mass, the NSDW, appear to be
produced in the strait, with production rates, between 77.6 and 79.93N, of about 0.2, 1.0 and
* Corresponding author. Fax: #33-1-44273805.
E-mail address: [email protected] (M.-N. Houssais)
Present address: Laboratoire d'OceH anographie Dynamique et de Climatologie, Paris, France.
0967-0645/99/$ - see front matter 1999 Elsevier Science Ltd. All rights reserved.
PII: S 0 9 6 7 - 0 6 4 5 ( 9 9 ) 0 0 0 1 7 - X
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P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1.7 Sv, respectively. A southward net fresh-water transport through the strait of about
2000 km yr\ (relative to a salinity of 34.93) is mainly due to the PW. The net heat transport
relative to !0.13C is northward, but undergoes a rapid northward decrease, suggesting an
area-averaged surface heat loss of 50}100 W m\ in the strait. 1999 Elsevier Science Ltd.
All rights reserved.
1. Introduction
In a companion paper (Schlichtholz and Houssais 1999, hereafter SH1), a threedimensional (3D) inverse method for estimating the geostrophic velocity "eld from
hydrographic data has been applied to the MIZEX 84 dataset to describe the
circulation and to estimate the volume transports in Fram Strait. As a continuation to
this work and using the same hydrographic dataset, the present paper presents
a water-mass analysis and gives estimates of the heat, fresh water and volume
transports, individually for each water mass and globally through the strait. The
analysis is based on a detailed volumetric census of the di!erent water masses
encountered in the strait, which is made possible by the accurate interpolation of the
hydrographic "elds provided by a method similar to the one used in SH1. The "elds
are then combined with the geostrophic velocity distribution derived in SH1 in order
to discuss the water mass circulation and transports in the strait and to provide
estimates of #uxes through the strait.
Fram Strait, the broad and deep trench separating Greenland and Spitsbergen
(Fig. 1), enables exchanges between the Nordic Seas, to the south, and the Arctic
Ocean, to the north, which are characterized by two highly contrasted hydrographic
regimes. The waters of polar and atlantic origins meet in Fram Strait where they are
roughly associated with distinct substantial in#ow and out#ow. This in#ow}out#ow
system is possibly encountered at all levels down to the sill depth (2600 m). In the
upper layers, the polar and atlantic water masses are separated by a sharp hydrographic front (the East Greenland Polar Front, or EGPF). In the deep layer, the
exchanges are, to some extent, controlled by the activity of the convection sites
renewing the deep water in the Nordic Seas and in the Arctic Ocean (Rudels, 1995).
Because of the di!erent characteristics of the deep-water masses thus formed to the
north and to the south of the strait, the deep exchange also should involve net
transports of heat and salt (Rudels and Quadfasel, 1991). On the other hand, the
in#ow of warm and salty Atlantic Water (AW), which occurs in Fram Strait in the
West Spitsbergen Current (WSC), is crucial for the "nal characteristics of the intermediate and deep-water masses formed in the Arctic Ocean (Rudels et al., 1994).
Together with the in#ow over the Barents Shelf, it constitutes the only relatively salty
water import to the Arctic Ocean (Aagaard and Carmack, 1989). It is also believed to
be one of the main heat sources for the Arctic Ocean excepts for ice export (Rudels,
1987). Similarly, the formation of dense, intermediate or deep-water masses to the
south of the strait should be controlled partly by the exchanges through Fram Strait,
in particular by the amount of sea ice and of fresh Polar Water (PW) advected from
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1139
Fig. 1. Bathymetry (in 100's of m) of Fram Strait. The dots indicate the station positions while the solid
and dashed lines correspond to the transport (Tables 2}4) and hydrographic (Figs. 4 and 5) sections,
respectively.
the Arctic Ocean in the East Greenland Current (EGC). The above features demonstrate the potential climatic impact of the exchanges through Fram Strait, although
very few estimates of the associated transports are available. More speci"cally,
detailed estimates of the individual contributions of the water masses to the exchanges
through the strait are needed to improve our understanding of the transformation
processes in the Arctic Mediterranean and as well as their variability. In this context,
the exchanges through Fram Strait cannot be reduced simply to a permanent system
of two opposite meridional in- and out#ows. Steady patterns of recirculation (see, e.g.,
SH1), as well as local modi"cations of the water masses through mixing or surface
interactions, take place in Fram Strait. These features have the potential to modify the
respective characteristics of the #ows entering or leaving the Arctic Ocean, and
ultimately the associated heat and salt #uxes. They can only be taken into account
through a 3D approach of the circulation and hydrography of the strait in which the
cross- as well as the along-strait variations can be reconstructed.
2. The data and the water-mass classi5cation
As in SH1, the present analysis is based on a selection of 342 CTD casts extracted
from a set of nearly 1500 deep casts taken in summer 1984 during the MIZEX 84
experiment. The data cover a domain extending between 77.15 and 81.153N across
Fram Strait (Fig. 1).
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P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
Table 1
Water mass characteristics
Number
Acronym
Temperature
Salinity
Percentage of t.v.
1
PW
AWw
AWF
AWc
MAW
AIW
7
8
UPDW
NSDWw
9
10
11
12
CBDW
NSDWc
EBDW
GSDW
S(34.7
S(34.4
S'34.91
34.4(S(34.91
S'34.91
34.4(S(34.91
34.7(S(34.9
34.9(S(34.92
34.7(S(34.9
34.9(S(34.92
34.9(S(34.92
S'34.92
34.9(S(34.92
S'34.92
34.7(S(34.92
2.88
2
3
4
5
6
h(03C
h'03C
h'23C
h'13C
03C(h(23C
03C(h(13C
!1.13C(h(!0.53C
!0.83C(h(03C
!0.53C(h(!03C
!0.83C(h(!0.53C
!0.53C(h(03C
!0.83C(h(!0.5
!1.13C(h(!0.83C
!1.13C(h(!0.83C
h(!1.13C
8.88
2.18
15.03
3.03
3.89
3.81
11.67
3.11
26.42
9.12
9.96
PW } Polar Water; AWw } warm Atlantic Water; AWF } fresh Atlantic Water; AWc } cold Atlantic
Water; MAW } Modi"ed Atlantic Water; AIW } Arctic Intermediate Water; UPDW } Upper Polar Deep
Water; NSDWw } warm Norwegian Sea Deep Water; CBDW } Canadian Basin Deep Water; NSDWc
} cold Norwegian Sea Deep Water; GSDW } Greenland Sea Deep Water; EBDW } Eurasian Basin Deep
Water.
t.v. } total volume of water in the analysed domain (see Fig. 2a).
if a salinity minimum is found in the range !1.13C(h(!0.53C; 34.7(S(34.9.
only if the mean h}S regression slope is negative.
if not AIW.
if not AIW nor UPDW.
In Fram Strait, in view of the complex interaction between the contrasted hydrographic regimes of the Nordic Seas and Arctic Ocean (see Section 1), a large variety of
water masses is found. In this section, we present a water-mass classi"cation for Fram
Strait in which 12 water masses are de"ned based to a large extent on criteria
proposed in the literature (Table 1 and Fig. 2). The distinct origin, geographical
distribution and circulation characterizing each of them is then discussed in the
following sections.
The AW is separated from the lighter PW by the isotherm h"03C and the
isohaline S"34.4 and from the denser Deep Water (DW) by the isotherm h"03C
(e.g., Rudels and Quadfasel, 1991). In the AW range, we select four modes, the warm
(AWw) and cold (AWc) modes with higher salinity (S'34.91), and the fresh (AWF)
and modi"ed (MAW) modes with lower salinity (34.4(S(34.91). The "rst two
modes are named according to Friedrich et al. (1995), although our AWw is con"ned
to the upper part of their salinity range and our AWF occupies the fresher range of
their AWw. The temperature limit between the AWw and the AWc, h"23C, corresponds to a sharp change in the slope of the h}S regression line (Fig. 2b). The upper
temperature limit for the MAW, h"13C, has been chosen after Rudels and
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1141
Fig. 2. h}S diagram constructed from the MIZEX 84 data for (a) the entire h}S range, (b) the Atlantic Water
range, and (c) the Deep Water range.
Quadfasel (1991). The water with the characteristics of our AWF is often referred to, in
the Nordic Seas, as the Arctic Surface Water (Swift, 1986).
The lighter modes in the DW range, due to their di!erent formation processes,
are not consistently de"ned by prescribed h}S ranges (Fig. 2c). Rather, following
Rudels et al. (1994), we identify the Upper Polar Deep Water (UPDW) as being
characterized by a negative slope of the h}S regression line in the warmer range
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!0.53C(h(03C. In Fram Strait, the UPDW meets the denser Arctic Intermediate
Water (AIW) de"ned by a salinity minimum (S(34.90) located in the temperature
range !1.13C(h(!0.53C. The h}S range of the AIW corresponds to the Upper
Arctic Intermediate Water as de"ned by Swift and Aagaard (1981) in the Nordic Seas.
Our AIW also includes all water with salinity above 34.90 and characterized by
a positive h}S regression slope.
The above de"nitions imply some overlap in the h}S space between the AIW and
the UPDW, as well as between them and a warm mode of the Norwegian Sea Deep
Water (NSDW), the NSDWw (Fig. 2c). This overlap also is observed in the real "eld
where active interleaving is made possible by the comparable densities of these water
masses. In view of these possible overlaps and in order to prevent the NSDWw from
overlaying the AIW at some locations, the water in the range !0.53C(h(03C,
which cannot be classi"ed as UPDW or AIW, is considered to be NSDWw. Similarly,
some of the water in the NSDWw range (34.90(S(34.92 and !0.83C(h(
!0.53C) is identi"ed as AIW if its h}S regression slope satis"es the above de"nition
of the AIW.
In the denser DW range, two salinity maxima (S'34.92), both signatures of haline
convection on the arctic shelves, are possibly found (Fig. 2c). The warmer salinity
maximum, between h"!0.83C and !0.53C, represents the Canadian Basin Deep
Water (CBDW), while the colder one, a quasi-isothermic, near bottom salinity jump,
identi"es the Eurasian Basin Deep Water (EBDW) (Aagaard et al., 1985; Rudels et al.,
1994). The range !1.13C(h(!0.83C and 34.90(S(34.92 de"nes the cold
mode of the Norwegian Sea Deep Water (NSDWc) and covers the characteristics of
the NSDW as de"ned by Swift and Koltermann (1988) in the Norwegian Sea. The
colder part of the water column (h(!1.13C), where it exists, is "lled with the
Greenland Sea Deep Water (GSDW) characterized by a downward salinity decrease
to values less than 34.90 (e.g., Swift, 1986).
3. Water mass distribution and volumetric census in summer 1984
From the selected MIZEX 84 data, the geographical distributions of the water
masses in summer 1984 have been obtained based on continuous estimates of the
large-scale time-mean potential temperature and salinity "elds over the domain
shown in Fig. 1. The interpolation scheme, derived from the method presented in SH1,
is outlined in the Appendix. Although the scheme is applied independently to the
temperature and salinity "elds and does not make use of any dynamic constraint, the
density distribution reconstructed from these estimates is quite consistent with the
density distribution estimated in SH1. As shown in SH1, the dynamics introduced in
the minimization problem to derive the geostrophic velocity "eld mainly constrain the
barotropic component of the #ow, while the baroclinic pressure gradients are only
slightly a!ected. The horizontal distributions of the reconstructed potential temperature and salinity "elds are shown in Fig. 3 for three depth levels, 20, 800 and 2000 m,
while vertical distributions along zonal sections in the southern (77.63N) and northern
(80.753N) part of the strait are presented in Figs. 4 and 5, respectively. These
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1143
Fig. 3. Horizontal distribution of the potential temperature in 3C (left) and salinity (right) based on the 3D
interpolated "elds at (a) and (b) 20 m, (c) and (d) 800 m, (e) and (f) 2000 m. The contour intervals are: (a) 0.5,
(b) 0.2, (c) 0.1, (d) 0.004, (e) 0.02, and (f) 0.002 of the corresponding units.
1144
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Fig. 4. Vertical distributions of the (a) potential temperature (in 3C) and (b) salinity based on the 3D
interpolated "elds along the parallel 77.63N. The contour intervals are: (a) 0.13C in the negative and 0.53C in
the positive temperature range; (b) 0.4 for S(34.6, 0.01 for 34.85(S(35.1 and 0.05 for S'35.1.
distributions very much resemble the original distributions (not shown here) except
for the disappearance of the mesoscale features, which are "ltered out by the interpolation. The large-scale horizontal contrast between the polar and the atlantic regimes,
each of them characterized by strong positive h}S correlations, is clearly evidenced.
The correlations are the stronger in the upper (Fig. 3a and b) and deep (Fig. 3e and f)
layers, but the two regimes have opposite characteristics. By contrast with the
southeastern part of the strait, in the northern and western parts, a relatively fresh and
cold mode characteristic of the PW is found in the upper layer while a comparatively
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1145
Fig. 5. Same as Fig. 4 except along the parallel 80.753N.
warm and salty mode representative of the Deep Arctic out#ow is identi"ed in the
deep layer. The vertical distributions (Figs. 4 and 5) illustrate the outcroping of the
h}S isopleths underlying the surface PW at the location of the EGPF and the gradual
westward shift of the front as it penetrates down to 200 m.
Fig. 6 shows the geographical distributions of the thickness of the layer occupied by
each water mass, while Fig. 7 presents the corresponding distributions of the depth of
the shallowest occurrence of each water mass in the water column. Only three water
masses outcrop at the surface, the PW being strictly a surface water and the other two,
the AWw and the AWF, reaching the surface only in some places (Fig. 7a}c). The PW
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Fig. 6. Horizontal distribution of the layer thickness (in m) for each of the water masses.
layer has a wedge-like structure (Fig. 6a). The layer thickness is maximum over the
Greenland Slope (150}200 m to the north) and decreases southward and eastward.
The AWw outcrops at the surface only in the southeastern part of the domain
(Fig. 7b), while to the north and to the west it sinks below the PW (Figs. 4 and 5) and is
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1147
Fig. 6. Continued.
present everywhere in the strait except over the Greenland Shelf (Fig. 6b) The layer
thickness reaches 400 m in some places over the Spitsbergen Slope. The AWF,
essentially present along the EGPF (Fig. 3a and b), spreads underneath the PW
as a less than 100 m-thick transition layer (Fig. 6c) between the PW and the AWw
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P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
(Figs. 4 and 5). As a consequence, the shallowest occurrence of the AWF(Fig. 7c)
is correlated with the layer thickness of the PW (Fig. 6a). Some AWF is also found at
the surface in the Boreas gyre and over the shelf to the southwest of Spitsbergen. The
most abundant of the three water masses is the AWw, which "lls as much as 9% of the
Fig. 7. Horizontal distribution of the shallowest occurrence (in m) of each of the water masses.
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1149
Fig. 7. Continued.
total volume of the domain. The PW and the AWF account for 2.9 and 2.2% of the
total volume, respectively.
The other two atlantic modes, the AWc and the MAW, are disconnected from the
atmosphere (Fig. 7d and e). The MAW is almost exclusively found over the Greenland
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Shelf and Slope (Fig. 6d and e), in agreement with its origin in the Arctic Ocean where
it appears as a permanent and omnipresent feature (Rudels et al., 1994). The relatively
weak temperature maximum characterizing the MAW in the northern part of our
domain suggests that the water mass is derived from the AW which enters the Arctic
Ocean through Fram Strait or across the Barents Sea, then #ows eastward along the
continental slope, and "nally returns along the Greenland Slope towards Fram Strait
(Anderson et al., 1994). The comparatively warm AWc and AWw are always found
east of the colder MAW (e.g., Fig. 5). The AWc spreads over the entire domain, except
over the shallow shelf areas, but preferential accumulation takes place in the western
part of the Strait, more speci"cally in the Lena Trough, in the sill region, and to the
south of the sill over the Greenland Slope. This distribution contrasts with the
location of the expected source region for the AWc. According to Aagaard et al. (1985)
and Boyd and D'Asaro (1994), the AWc would be formed by winter cooling of the
AWw, either to the south of the strait or in the WSC within the strait. Although the
AWc and the MAW layers have a comparable maximum thickness (exceeding 500 m),
the former contributes "ve times more to the overall volume. In fact, the AWc is the
most abundant (15% of the total volume) of the upper water masses, and the second
most abundant, after the NSDWc, of all the water masses.
The two lighter modes of the DW, the AIW and the UPDW each contribute ca. 4%
of the total volume. The AIW, formed by winter cooling and sinking of the surface
water to intermediate depths in the Greenland Sea gyres (Swift, 1986), is con"ned, in
our domain, to the Boreas Basin. It appears as a relatively shallow salinity minimum
at about 300 m in the centre of the gyre, which spreads below the AWc towards the
periphery (e.g., Fig. 4). The maximum layer thickness exceeds 700 m in the Central
Basin with an averaged layer depth of nearly 600 m (Fig. 6f and 7f). The UPDW,
formed in the Arctic Ocean by interaction of sinking plumes due to ice formation on
the shelves with in#owing AW (Rudels et al., 1994), is identi"ed all along the
Greenland Slope and in the Lena Trough (Fig. 6g). Its maximum layer thickness
exceeds 700 m in the northern part of its domain. The rapid southward decrease of its
volume must be due partly to local modi"cation by interaction with the NSDW and
the AIW, partly to the di$culties in discriminating between these three water masses
based on their h}S characteristics or to a limited number of slope stations in the
southwestern part of our domain (Fig. 1). Indeed, some water with the same characteristics as the UPDW is found farther south along the slope, in the Greenland Sea
(Strass et al., 1993). Depending on the distance from the Greenland Shelf, the UPDW
is overlaid either by the MAW or by the AWc (Figs. 6 and 7). A remarkable feature
associated with the UPDW is a salinity minimum found at h"!0.13C (Fig. 2c),
below the temperature maximum characterizing the AW (Fig. 5) and clearly identi"ed
at 800 m (Fig. 3d). The salinity minimum also has been observed in the Eurasian Basin
in the vicinity of Fram Strait, where it is interpreted as the result of mixing of the AW
recirculating north of the strait with the intermediate water column advected from the
Canadian Basin (Rudels et al., 1994).
As for the UPDW, the distribution of the CBDW, with water concentrated along
the East Greenland Slope and in the Lena Trough, is indicative of a deep Arctic Ocean
out#ow (Fig. 6i). These two water masses also represent similar volumes (ca. 3% of the
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1151
total volume). Still, compared with the UPDW, the maximum thickness of the CBDW
layer is shifted eastward (Fig. 6g and i) and its averaged depth (1500 m) is shifted
downward (Fig. 6g and i, Fig. 7g and i). The CBDW accumulates in the Lena Trough
where the layer is thicker than 500 m but some also is encountered in the northeastern
part of the Strait, in the Litke Trough. The deeper (the most part below 1500 m)
EBDW, although more abundant (9.1% of the total volume) than the CBDW, does
not penetrate as far to the south. Rather, it stays in the northern part of the strait,
mostly in the Lena Trough as a 1500-m thick layer, or in the Molloy Deep area as
a 1000-m thick layer overlaying the colder and fresher products from the south
(Fig. 6k). Although the 2600-m sill prevents most of the EBDW from entering the
Greenland Sea, a minor portion of the water mass is also found to the south of the sill
below 2000 m along the Greenland Slope.
The NSDW is by far the most abundant water mass in Fram Strait, contributing by
almost 40% to the total volume. Its cold mode (NSDWc) is the more important since it
"lls more than 25% of volume and is present everywhere in the strait, forming a more
than 1000 m-thick layer in the southern and central parts of the strait (Fig. 6j). The
NSDWc accumulates over the bottom topographic features in the eastern part of the
strait (the Knipovich Ridge, the Molloy Deep and the Molloy and Spitsbergen Fracture
Zones) with maximum thicknesses greater than 1750 m. The upper boundary of the layer
deepens towards the western side of the strait where it is capped by the deep out#ow of
CBDW from the Arctic Ocean. The NSDWw, although less abundant (12% of the total
volume), also spreads over most of the strait, except in the Boreas Basin where it is
replaced by the AIW (Fig. 6h). As for the NSDWc, there is an accumulation of the
NSDWw in the Molloy Deep, but the overall layer thickness distribution is quite
di!erent, with additional large volumes of NSDWw encountered to the north and to the
west of the Boreas Basin rather than on the eastern side of the strait. These accumulations
suggest possible transformations into NSDWw outside the Boreas Basin. In particular,
while all the other deep-water masses lie below 1000 m, the NSDWw outcrops at depths
as shallow as 600 m over the Greenland Slope (Fig. 7h), a feature which may indicate that
the lower East Greenland Slope is a preferred formation site for this water mass.
The GSDW, produced through intense surface cooling in the Greenland Sea gyres
(e.g., Clarke et al., 1990), is the second important deep water mass after the NSDW in
Fram Strait. It accounts for almost 10% of the total volume and is almost exclusively
found south of the sill, in its source region, the Boreas Basin. There, it forms a more
than 1000 m-thick layer below 2000 m (Fig. 6l and 7l). As the coldest and densest
water mass, it also "lls the deeper few hundred meters of the Molloy Deep. It should
be noted that the volume estimate and the distribution of the GSDW (and of the AIW)
are only approximative since these water masses are the more abundant in an area
where only a limited number of hydrographic pro"les was available (Fig. 1).
4. A 3D view of the water-mass circulation and transformations
The spatial distribution of the water masses is essentially determined by their
circulation and by the transformation processes that take place outside as well as
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P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
locally in the strait. In this section, the water-mass circulation is discussed after
combining the detailed large-scale view of the Fram Strait hydrography presented in
Section 3 with an estimate of the velocity "eld obtained in SH1. The selected velocity
"eld corresponds to solution p in SH1, that is to a compromise between solutions
that better perform in the EGC (e.g., p ) and others that are more realistic in the WSC
(e.g., p ). The horizontal distributions of the velocity integrated over the depth interval
occupied by each water mass are presented (Fig. 8). The #ow distributions are then
integrated across the Strait in order to estimate the net transports of the water masses
through the Strait. To illustrate the robustness of our transport estimates despite
some uncertainty on the #ow solution (see SH1), the averaged transport and its
standard deviation calculated between the six solutions (p }p ) selected in SH1 and
corresponding to di!erent values of the inverse model parameters are also listed in
Table 2. Also given are the transports for two additional solutions (p , p ) best
illustrating the sensitivity of the #ow solution to these parameters. The transports are
calculated through three zonal sections crossing the Strait at 77.6, 78.9 and 79.93N
and reaching onto the shelves on both sides of the strait. These sections divide the
domain into a central and a southern box (Fig. 1) so that the net convergence (resp.
divergence) of the #ow of any given water mass in a box can be interpreted in terms of
a mean (over the period under consideration) transformation (resp. production) rate of
this water mass within the box.
4.1. Water mass circulation, transports and transformations
In this analysis, the di!erent currents are named according to SH1 (see
Fig. 19 therein) where more details about the 3D distribution of these currents can be
found.
4.1.1. Polar water
The horizontal extent of the PW wedge is closely related to the southward #ow of
the EGC and the Polar Current (PC), which together make a broad #ow entering
northern Fram Strait directly from the Arctic Ocean, with a small contribution
supplied by the Westwind Trough Current (WTC) from the Greenland Shelf at 803N
(Fig. 8a). Due to interaction with the WSC system, the PW layer gets thinner eastward
from the Greenland Slope, reaching a thickness of less than 50 m in the PC, west of the
Yermak Plateau. A small portion of the PW from the PC takes part in a cyclonic
recirculation over the Yermak Plateau, but the major portion is transported in the
southwestward Polar Front Current (PFC) and joins the EGC. The horizontal extent
of the PW wedge therefore rapidly decreases southward, as the #ow gets trapped by
rotation above the East Greenland Shelf and Slope. This feature, noted by Aagaard
et al. (1985) and Foldvik et al. (1988), has been further supported by laboratory
experiments (Hunkins and Whitehead, 1992). Still, a local excursion of the PW
towards the southeast appears at about 79.53N (Fig. 6a) and is related to the presence of a pronounced eddy in the Molloy Deep region (Fig. 8a), a typical summer
feature already mentioned by Gascard et al. (1988) as most probably related to an ice
tongue.
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1153
The volume budget for the PW suggests that only little, if any, transformation of
this water mass occurs within Fram Strait. At 79.93N, the southward net #ow of PW is
equal to 1.1$0.1 Sv. At 78.93N, it is somewhat smaller (0.9 Sv), but the di!erence,
Fig. 8. Horizontal distribution of the transports (in m s\) associated with the water masses. Also shown
is the bottom topography (in 100's of m).
1154
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
Fig. 8. Continued.
partly compensated by the out#ow of 0.1 Sv towards the shelf, is too small to be
signi"cant. In the southern box, a small de"cit of PW is found, which is due to
a meridional #ow convergence of 0.6 Sv slightly overcompensated by the out#ow of
0.7 Sv onto the Greenland Shelf. Our estimate of the PW out#ow from the Arctic
1
!1.20
!1.26
!1.03
!1.13
!0.12
!0.99
!0.85
!0.97
!0.94
0.07
!0.36
!0.26
!0.36
!0.32
0.04
WM
p
p
p
p
p
p
p
p
p
p
p
p
p
p
p
!0.15
!0.13
!0.17
!0.16
0.02
!0.26
!0.06
!0.21
!0.20
0.09
!0.36
!0.25
!0.33
!0.32
0.04
0.11
0.53
0.38
0.23
0.28
0.23
1.14
0.97
0.67
0.38
3
!0.15
0.10
!0.03
!0.12
0.28
2
!1.79
!0.67
!1.46
!1.37
0.46
!1.63
!1.21
!1.50
!1.46
0.24
!1.09
!0.84
!0.82
!0.99
0.25
4
0.00
0.00
!0.01
0.00
0.00
!0.33
!0.11
!0.24
!0.23
0.10
!0.54
!0.62
!0.57
!0.53
0.17
5
7
!0.98
!0.96
!1.37
!1.02
0.33
!0.79
!0.76
!0.92
!0.75
0.16
!0.18
!0.09
!0.10
!0.13
0.06
79.93N
!0.03
!0.04
0.00
!0.02
0.02
78.93N
0.00
0.00
0.00
0.00
0.00
6
77.63N
!0.75
!0.65
!0.13
!0.62
0.45
!1.33
!0.56
!1.54
!1.09
0.51
!0.74
!0.48
!0.66
!0.68
0.23
!0.60
!0.36
!0.35
!0.47
0.26
8
!0.09
!0.07
!0.10
!0.09
0.04
!0.36
!0.44
!0.45
!0.37
0.10
!0.35
!0.34
!0.40
!0.35
0.07
9
!1.03
!0.37
!0.59
!0.96
0.74
!0.77
!0.18
!0.53
!0.59
0.42
!0.04
0.37
0.28
0.13
0.33
10
!0.06
!0.04
!0.14
!0.07
0.04
!0.14
!0.19
!0.13
!0.14
0.04
!0.34
!0.31
!0.37
!0.33
0.10
11
0.00
0.00
0.00
0.00
0.00
!0.30
!0.01
0.35
!0.11
0.58
!0.04
!0.04
!0.04
!0.04
0.01
12
Table 2
Net meridional volume transport through Fram Strait (in 10 m s\) associated with each of the 12 water masses (WM) de"ned in Table 1 for three partcicular
solutions (p , p and p ) as well as the average (p ) and standard deviation (p ) for six solutions (p }p ) of the geostrophic streamfunction (see SH1) at three
latitudes (77.6, 78.9 and 79.93N). Positive values indicate a northward transport.
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1155
1156
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
Ocean through Fram Strait is comparable with the 0.8 Sv calculated by Houssais et al.
(1995) or with the 0.5}0.9 Sv obtained by Rudels (1987), all using an inverse model,
and is also in good agreement with estimates by Manley et al. (1987) from a two-layer
boundary current model of the EGC or by Foldvik et al. (1988) based on current
meter measurements in the EGC.
4.1.2. Atlantic Waterwarm
Several westward recirculating branches originating in the WSC have been reported in the literature (e.g., Manley, 1995; Gascard et al., 1995). In Fig. 6b, a signature
of such a recirculation is clearly identi"ed to the south of the sill, between 78 and
793N, as a westward protrusion of AWw. The tongue, thicker than 100 m, is correlated
with a #ow that reaches beyond the Greenwich meridian and then, embedded in the
Return Atlantic Current (RAC), joins the EGC (Fig. 8b). The velocity distribution in
the AWw layer also con"rms the existence of two branches of AWw associated with
the WSC (see, e.g., SH1), a o!shore branch proceeding northward along the western
#ank of the Yermak Plateau, and a coastal branch #owing along the shelf break and
identi"ed as the North Spitsbergen Current (NSC) (Fig. 8b). Each branch is characterized by an accumulation of AWw over a depth range of more than 250 m (Fig. 6b) and
corresponds to a high temperature and salinity core identi"ed on each side of the
Yermak Plateau (Fig. 5). The subsequent fate of the o!shore branch of AWw (Fig. 8b)
also corresponds to the circulation scheme described in SH1. Accordingly, the AWw
contributes to a westward recirculation in the Spitsbergen Fracture Zone Current
(SFZC) and to a northward #ow over the Yermak Plateau (the Yermak Plateau
Current, or YPC) and over the upper slope to the west of the plateau (the Yermak
Slope Current, or YSC). The northward #ow apparently recirculates, at least partly,
farther north, since a southward #ow of AWw is identi"ed over the lower slope to the
west of the plateau, as a layer underlying the PW layer of the PC. A major portion of
this recirculation, however, turns eastward to the south of the plateau and therefore
closes a subsurface cyclonic gyre which is an extension of the surface gyre identi"ed
over the Yermak Plateau in the PW layer.
The net transport of AWw is northward through the two southernmost sections
across the strait but fairly small (0.2 Sv on the average) at 78.93N. At 79.93N, it is
northward only in p , one of the most realistic solutions for the WSC. The AWw
volume budgets in the southern and central boxes consistently indicate a net accumulation of this water mass within the strait. On an average, this accumulation corresponds to a transformation rate of about 0.4 Sv in each of these boxes. These budgets
include the net #ow through the eastern boundary of the strait, which is negligible in
all cases. Although the overall loss of AWw should participate to production of some
AWF in the strait, it is likely to contribute to a larger extent to formation of AWc (see
below).
4.1.3. Atlantic Water fresh
As a transition layer between the PW and the AWw, the main body of the AWF is
either transported southward by the PFC, with a small contribution added by the
RAC, or participates in the cyclonic circulation over the Yermak Plateau (Fig. 8c).
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1157
That part of the AWF layer outcroping over the shelf to the southwest of Spitsbergen
(Fig. 7c) is probably supplied by the East Spitsbergen Current and therefore participates in the anticyclonic surface circulation around the archipelago (Hopkins, 1991).
An along-shelf #ow of AWF is indeed observed in the WSC and farther north in the
NSC (Fig. 8c).
The net transport of AWF increases southward indicating that some of this water
mass is likely to be formed locally in the strait. On the average, about 0.15, 0.2 and
0.3 Sv of AWF #ow through the sections at 79.9, 78.9 and 77.63N, respectively. When
these meridional transports are combined with an averaged #ow of 0.1 Sv of AWF
towards the Spitsbergen coast in the central box, a net #ow divergence of approximately 0.1 and 0.15 Sv is obtained in the southern and in the central box, respectively.
These production rates are signi"cant when compared with the smaller standard
deviation on the individual transport estimates and are consistent with a postulated
local formation of AWF from AWw in Fram Strait. Although the volume of PW does
not appear to undergo signi"cant changes over the Strait, such production of AWF
must occur through interaction of the AWw with PW or with sea ice. According to the
AWw budget in the strait (see above), it is indeed possible that the source water mass
for the production of AWF in Fram Strait be the AWw.
4.1.4. Modixed Atlantic Water
The MAW, together with the di!erent recirculating branches of AWw and AWc,
contributes to the warm, intermediate depth core underlying the PW in the EGC
system (Fig. 8e). Almost everywhere in this current, a mixture of the three Atlantic
water masses is present which leads to a large spreading of the temperature maximum
characterizing this core (Fig. 2c, Figs. 4 and 5).
The net out#ow of MAW from the Arctic Ocean at 79.93N is about 0.5 Sv on
average, but the southward transport decreases as the water mass proceeds southward
in the strait and vanishes at the southernmost section. Taking into account a net #ow
of 0.3 Sv of MAW towards the Greenland Shelf, the required transformation rate of
this water mass in the strait is only 0.2 Sv, that is not much larger than the uncertainty
on the out#ow of MAW from the Arctic Ocean. Moreover, part of the transformation
may be an artefact of the interpolation since there is a lack of data in the southwestern
corner of our domain (Fig. 1a). Still, a transformation of MAW by mixing with cores
of warmer and saltier AWc recirculated from the WSC could be realistic.
4.1.5. Atlantic Water cold
Some AWc enters Fram Strait below the AWw in the WSC (Fig. 8d). The AWc
constitutes the main body of the southward recirculating AW as indicated by its layer
thickness distribution (Fig. 6d) and by the temperature of the subsurface salty core
sitting over the Greenland Slope at 77.63N (Fig. 4). The RAC is associated with
a strong #ow of AWc (Fig. 8d). As opposed to the AWw #ow (Fig. 8b), the #ow is
stronger on the western side of the RAC, suggesting local transformation of AWw into
AWc. In particular, the recirculation of AWw in the SFZC, north of the Molly Deep
area, might be associated with a gradual transformation of the AWw into AWc. The
possible existence of a northern recirculating branch has already been mentioned by
1158
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
Quadfasel et al. (1987) as being the result of the large-scale potential vorticity
conservation in the area. The role of the bottom topography is indeed evidenced by
the shape of the warm and salty tongue which follows the Spitsbergen Fracture Zone
at 800 m (Fig. 3c and d), i.e. at the deepest level reached by the AWc in this area
(Fig. 6d and Fig. 7d). The large amount of AWc in the Lena Trough is associated with
the southward #ow of the PC, which transfers the AWc to the PFC and, continuing
southeastward along the Spitsbergen Slope, also to the RAC (Fig. 8d). Some of AWc
from the PC also is recirculated northward over the Yermak Plateau underneath the
AWw although, according to the opposite gradients of the respective layer thicknesses
in Fig. 6b and d, most of the AWc found in the YPC and in the YSC should rather
result from local transformation of AWw into AWc. However, the large amount of
AWc in the PC suggests that the "nal transformation of AWw into AWc must occur
north of the Fram Strait. Some AWw with h'23C has indeed been observed north of
823N (Muench et al., 1992).
At any latitude the net transport of AWc is southward, amounting to 1 Sv on the
average at 79.93N. The AW budget is closed to within 0.1 Sv in the central box of the
domain where the averaged net transport divergence of 0.5 Sv of AWc is consistent
with the simultaneous loss of 0.4 Sv of AWw in this box as a result of direct
transformation of AWw into AWc in this area. In the southern box, on the other hand,
the volume accumulation of AWc is not signi"cant in view of the uncertainty on the
estimates.
4.1.6. Arctic Intermediate Water
The AIW participates in the cyclonic circulation of the Boreas Basin gyre (BBG)
(Fig. 8f). Our analysis suggests that there is no AIW penetrating northward into the
Arctic Ocean although it is possible that some AIW joins the northward #ow of the
WSC but, on its way, mixes with the saltier surrounding water masses and acquires
the characteristics of the NSDW.
A southward net transport of AIW of 0.6 Sv occurs at 77.63N. The out#ow is not fed
from farther north, which implies an equivalent, unrealistic in summer, production of
this water mass within the strait. Since most of the AIW lies directly underneath the
AWc in the Boreas Basin, erroneous identi"cation as AIW of some of the AWc is
likely. This error would tend to underestimate the southward branch of the AWc #ow
through the section at 77.63N, while producing the spurious AIW out#ow at the same
latitude. Another possible error on the net transport of AIW at 77.63N may be due to
the fairly strong individual in- and out#ows in the BBG.
4.1.7. Upper Polar Deep Water
The UPDW is carried southward in the EGC and in the PC, then turns southwestward in the PFC and merges farther south with the EGC (Fig. 8g). The southward
#ow of UPDW is ultimately con"ned to a thin along-slope boundary current (Fig. 6g).
The net transport of UPDW decreases from about 1 Sv at 79.93N to 0.1 Sv on the
average at 77.63N implying an important transformation into other deep-water
masses. Indeed, an averaged production of 0.6 Sv of NSDWw is identi"ed between
79.9 and 77.63N (see below).
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1159
4.1.8. Norwegian Sea Deep Water warm
The overall circulation pattern of the NSDWw resembles that of the AWc (Fig. 8h
and d). The NSDWw mainly enters the strait from the north, in the PC and in the
EGC, with only a small amount coming from the south in the WSC. The NSDWw
from the PC feeds the PFC and the RAC and participates in the clockwise circulation
to the east of the Molloy Fracture Zone. The NSDWw from the WSC mainly
recirculates in the RAC. All these sources ultimately feed the EGC system and
contribute to a southward net exit of the water mass to the Greenland Sea. Only
a small portion of NSDWw seems to penetrate northward along the eastern slope of
the strait into the Arctic Ocean.
The net transport of NSDWw is southward at any latitude but increases southward
from 0.5 to 1.1 Sv on an average between 79.9 and 77.63N. Still, these values and the
circulation scheme discussed above are somewhat uncertain since a major portion of
the NSDWw found over the Greenland Slope, to the north, or in the PC, may rather
be some UPDW or CBDW with slightly modi"ed salinity, while some of the NSDWw
found in the RAC may be AIW which has lost its original characteristics on the rim of
the Boreas gyre. In any case, the divergence of the NSDWw transports in the strait is
well correlated with the loss of UPDW, suggesting a local transformation of
some UPDW into NSDWw (see above). Such a transformation would be an
alternative to the scenario occurring in the Nordic Seas where the NSDWw is thought
to be formed by modi"cation of the old NSDWc during its renewal phase (Swift,
1986).
4.1.9. Arctic Ocean Deep Water: Canadian Basin Deep Water and Eurasian Basin Deep
Water
As with the UPDW, the southward #ow of CBDW hangs as a boundary current
over the Greenland Slope and gets narrow south of 79.53N (Fig. 8i). The current #ows
at about 1500 dbar (Fig. 6i and 7i), a level that roughly corresponds to the compensation pressure at which the relatively warm and salty core of CBDW, in contact with
the colder and fresher water column of the Greenland Sea, is constrained by the
thermobaric e!ect (Aagaard et al., 1985). The EBDW enters the strait in the Lena
Trough and, from there, proceeds southwestward. The major portion reaches the
Molloy Deep area where it recirculates northward in a weak cyclonic circulation
while a smaller part follows the East Greenland Slope and passes the sill down to the
Greenland Sea (Fig. 8k). This southward extension, already reported in the literature
(Aagaard et al., 1991), is identi"ed in the southern salinity section (Fig. 4b), but as
a weak signature probably due to the smoothing e!ect of our analysis.
The southward decrease of the transports of EBDW and CBDW results in a transformation rate of 0.5 Sv for the two water massses altogether. If the transports at
78.93N are chosen as our estimate of the deep out#ow from the Arctic Ocean, a value
of approximately 1.3 Sv shared between the UPDW, the CBDW and the EBDW is
obtained, which is consistent with the deep transport of 2 Sv postulated by Aagaard
et al. (1991) based on hydrographic data from June 1987. Indeed, the latter estimate
also takes into account the NSDW, which is formed north of 78.93N at a rate
estimated in the present study to about 0.9 Sv.
1160
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
4.1.10. Norwegian Sea Deep Water cold
Previous studies suggest that part of the NSDWc formed through mixing between
the EBDW and the GSDW along the periphery of the Greenland Sea gyres recirculates towards Fram Strait (e.g., Smethie et al., 1988). According to the present analysis,
a large amount of NSDWc overlying the GSDW participates in the cyclonic circulation of the BBG and contributes to a southern branch of the RAC (Fig. 8j). An even
more considerable fraction of the NSDWc enters Fram Strait in the deep WSC, with
a branch recirculating eastward and then southward along the Spitsbergen Slope, and
another one, to the west of the Knipovich Ridge, proceeding northward along the
topography. The latter branch mostly recirculates in the SFZC, and only a small
portion continues its northward course to the Arctic Ocean over the western slope of
the Yermak Plateau in the YSC. Some water with the characteristics of the NSDWc
enters the domain from the north along the Greenland Slope but is most probably
some EBDW.
The net transport of NSDWc is generally southward and, on the average, accounts
for about a half of the 1.3 Sv of the total NSDW net #ow at 78.93N. As for the
NSDWw, the net transport of NSDWc increases southward implying an averaged
production of 1.1 Sv of this water mass between 79.9 and 77.63N and making a total
formation rate of NSDW of 1.7 Sv. Possible formation of NSDW in Fram Strait has
been suggested by Smethie et al. (1988) and by Aagaard et al. (1991). Our estimate of
the production rate is fairly large but consistent with the required net consumption of
1.4 Sv of Arctic Ocean Deep Waters (UPDW, EBDW and CBDW) between the same
sections. The consistency implies that the new NSDW is formed through mixing of
some old NSDW characterizing the deep-water column coming from the Nordic Seas
with these Arctic Ocean Deep Waters. The consistency also holds in the central box
where an averaged production of about 0.9 Sv of NSDW coincides with a transformation rate of 0.5 Sv of the Arctic Ocean Deep Waters, and the di!erence, according to
SH1, can be attributed to the overall mass imbalance of 0.5 Sv in the box.
4.1.11. Greenland Sea Deep Water
The circulation in the GSDW is cyclonic as a consequence of the substantial
barotropic component in the BBG (Fig. 8l). A very small portion of GSDW passes
northward and is trapped with some NSDWc below the EBDW in the near bottom
cyclonic circulation of the Molloy Deep. Finally, some GSDW entering Fram Strait in
the WSC completely recirculates southward in the area of the Knipovich Ridge. The
meridional transport of GSDW is in balance on an average, but the standard
deviation between all the solutions is fairly large since the net transport is made by the
di!erence between an in#ow and an out#ow of similar magnitudes. Only solution
p produces a net accumulation of the water mass in the Strait, which should be
compensated by a transformation of GSDW into NSDW. According to p , the
corresponding production rate would be 0.4 Sv.
4.1.12. Overall budget
According to the above transports estimates, the averaged net out#ow from the
Arctic Ocean of 5.2 Sv at 78.93N, as estimated by SH1, appears to be almost equally
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1161
shared between a deep (2.6 Sv) and a surface and intermediate (2.6 Sv) out#ow
(Table 2). Separating the deep out#ow into a component from the Nordic Seas
(NSDW) and a component from the Arctic Ocean, each contributes by an equal
amount of 1.3 Sv to this out#ow. In the upper layer, both the PW (0.9 Sv) and the
products derived from transformation of the AW inside and north of the strait (mostly
AWc and to a less extent MAW) contribute altogether to the upper out#ow of 2.6 Sv.
The out#ow of 1.7 Sv of AW products is realistic when compared with independent
estimates based on current meter measurements, which suggest a possible in#ow of
2 Sv of AW to the Arctic Ocean over the Barents Sea (Loeng et al., 1993). The in#ow of
AWw to the Arctic Ocean at 78.93N is fairly small (0.2 Sv on an average).
At 79.93N, the averaged net out#ow of all AW products from the Arctic Ocean is
about 1.8 Sv. This agrees with previous results, suggesting that the transport of AW in
the EGC exceeds that in the WSC (Aagaard and Coachman, 1968). However, the net
AW out#ow at 77.63N is only 1 Sv while the net out#ow towards the shelves amounts
to 0.4 Sv, indicating some transformation of these water masses in the strait. Most of
the transformation (0.5 Sv) occurs in the southern box and should feed the deep-water
range (e.g. the NSDWW), but the rate may be somewhat overestimated due to an
underestimate of the out#ow at 77.63N (see Arctic Intermediate Water above). Converting 0.4 Sv of the net production of 0.6 Sv of AIW in the southern box into an
equivalent production of AWC, the averaged de"cit of 0.4 Sv of AWW in this box is
consistently explained by the transformation of this water mass into AWC.
4.2. Meridional heat and fresh-water transports
The net meridional transports of heat and fresh water at 77.6, 78.9 and 79.93N, and
the corresponding imbalances in the southern and central box, are given in Table 3 for
solutions p , p and p . The contribution of each water mass to these transports is also
given in Table 4 for the northernmost and southermost sections. For the heat
Table 3
Net meridional transport of heat and fresh water at 77.6, 78.9 and 79.93N through Fram Strait, and
corresponding imbalances between 77.6 and 78.93N, and between 78.9 and 79.93N, for three solutions (p ,
p and p ) of the geostrophic streamfunction. The heat and fresh water transports are relative to !0.13C
and 34.93, respectively. Positive values indicate a northward transport or a local convergence of the
transports through the boundaries.
Heat #ux (10 W)
p
79.93N
78.93N
77.63N
78.9}79.93N
77.6}78.93N
p
Fresh water #ux (km yr\)
p
p
p
p
0.1
4.3
5.8
2.4
10.9
19.8
!0.2
8.5
14.4
!2042
!1890
!592
!2156
!1691
!528
!1955
!1878
!615
!3.6
!4.5
!7.9
!9.5
!5.4
!5.0
!28
123
!136
!351
181
96
4.7
5.1
4.1
!1.6
!1.1
!1.6
!1956.5
!2044.1
!1691.0
!436.4
!318.9
!440.0
p
p
p
p
p
p
p
p
p
1
p
p
p
WM
!11.6
!7.9
!12.2
2.1
!16.1
!6.5
!66.3
!42.6
!61.5
!2.4
!0.9
!1.3
8.5
20.8
19.6
!65.6
!147.4
!139.9
!0.5
!0.2
!0.6
3
!1.4
1.3
!0.1
2
!2.3
!2.2
!2.7
!0.7
!0.5
1.3
Fresh water #ux (km yr\) at 79.93N
!66.9
!0.9
!16.1
!6.5
!75.8
!1.3
!16.3
!5.0
!67.7
0.0
!22.6
!5.4
Fresh water #ux (km yr\) at 77.63N
!0.3
!20.6
!3.4
!19.7
!0.1
!11.1
!1.7
!7.7
!1.3
3.2
!1.9
!23.6
19.8
10.6
11.1
44.1
9.8
37.9
!8.3
!2.7
!6.7
!5.3
!4.2
!4.6
0.8
0.8
0.9
9
0.2
0.2
0.2
8
2.4
1.1
2.8
7
Heat #ux (10 W) at 77.63N
0.0
1.7
0.2
0.0
1.1
0.1
!0.1
0.0
0.1
6
1.1
0.6
0.8
5
Heat #ux (10 W) at 79.93N
!1.3
0.1
0.8
!1.4
0.1
0.8
!1.5
0.0
1.1
4
!16.0
!7.6
!7.6
!0.5
4.3
3.5
3.6
1.2
2.0
0.1
!1.3
!1.0
10
!0.6
!0.4
9.0
!2.5
!2.3
!2.5
0.2
0.1
0.9
1.1
1.0
1.2
11
!7.2
0.1
9.4
0.0
0.0
0.0
1.3
0.1
!1.5
0.0
0.0
0.0
12
Table 4
Net meridional transport of heat and fresh water at 77.6 and 79.93N through Fram Strait, associated with each of the 12 water masses (WM) de"ned in Table 1 for
three solutions (p , p and p ) of the geostrophic streamfunction. The heat and fresh water transports are relative to !0.13C and 34.93, respectively. Positive
values indicate a northward transport.
1162
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1163
transport, the reference temperature is !0.13C, which, according to Aagaard and
Greisman (1975), corresponds to the mean temperature of the Arctic Ocean out#ow.
The reference salinity for the fresh-water transport is 34.93 as in Aagaard and
Carmack (1989).
The net heat transport through Fram Strait is northward. At 79.93N, according to
our estimates, it is mainly achieved by the cold out#ow of surface and deep waters
originating in the Arctic Ocean. Only in p does the AWw in#ow add a signi"cant
contribution to the heat input to the Arctic Ocean. In all cases, the heat transport
rapidly decreases northward in the strait, from 5 to 20 TW (10 W) at 77.63N to 0 to
2 TW at 79.93N, depending on the solution. The resulting net heat #ux to the Arctic
Ocean is very small. A small value of 6 TW also was obtained by Houssais et al. (1995)
at about 803N. Still, our estimate is smaller than the 18 TW obtained by Rudels (1987)
through a section running between 79 and 803N. If the heat input to the Arctic Ocean
associated with the ice out#ow in the EGC can indeed be estimated to 34 TW as
proposed by Aagaard and Greisman (1975), our result suggests that, at least in
summer, this out#ow is certainly the major heat source for the Arctic Ocean, well
before the AW contribution. On the other hand, the heat-#ux convergence of approximately 5}10 TW, found in either the central box or the southern box, is of comparable magnitude with the net heat transport itself and is therefore a real feature. The
convergence is indeed correlated with the northward heat loss experienced by the
AWw. For instance, in p , the net heat transport due to the #ow of AWw is equal to
20 TW at 77.63N and decreases to 1 TW at 79.93N. The heat loss from the AWw can
be attributed either to exchanges at the sea surface or to mass changes due to local
transformation of AWw into AWc or AWF. In the former case, the corresponding
area-averaged surface heat loss would be about 200 W m\. The overall heat loss of
17 TW from all water masses in p in the same latitude range is consistent with the
24 TW found by Houssais et al. (1995) in a smaller latitude band somewhat shifted to
the north. Assuming a ratio of 1 : 1 between the heat loss to the atmosphere and the
heat loss to the ice, as proposed by Houssais et al., and considering that the marginal
ice zone roughly covers 20% of the strait area, the ocean heat #ux underneath the ice
would be 125}250 W m\. This estimate compares well with measurements of the
basal melting rate made by Josberger (1987) in the summer marginal ice zone in 1983
in Fram Strait. Maximum melt rates of approximately 0.5 m day\ have been reported, which roughly correspond to an oceanic heat #ux to the ice of 170 W m\.
Indirect estimates by Untersteiner (1988) suggest a mean heat loss of approximately
300 W m\ for the Atlantic layer underneath an advancing ice edge northwest of
Spitsbergen.
A southward net fresh-water transport of 2000 km yr\ at 79.93N is found in all
solutions and is mainly associated with the PW out#ow. All other contributions to
this export are two to four orders of magnitude smaller. Our estimate is somewhat
larger than the 1160 km yr\ proposed by Aagaard and Carmack (1989) but, as
a yearly mean, the latter may underestimate the summer situation. Our estimate is
comparable with those proposed by Houssais et al. (1995) or Friedrich et al. (1996)
based on a subset of the same MIZEX 84 data. The drastic diminution of the
meridional fresh-water export between the northern and southern sections is almost
1164
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
entirely explained by a fresh-water export to the Greenland Shelf associated with
a PW #ow. The fresh-water imbalance in the strait is therefore less than 100 km yr\
(Table 4). The second important contribution to the fresh-water export at 77.63N is
due to the northward #ow of salty AWw but, due to the important transformation of
this water mass in the strait, this contribution becomes negligible as the AWw enters
the Arctic Ocean at 79.93N.
5. Summary
A unique 3D view of the large-scale time-mean summer water mass distribution in
Fram Strait has been extracted from the MIZEX 84 CTD data using an interpolation
scheme applied to the potential temperature and salinity "elds. The interpolated
hydrographic "elds have been then combined with the geostrophic #ow estimated
from the same hydrographic data to analyse the circulation for the di!erent water
masses encountered in the strait and to estimate possible transformation rates as well
as the associated volume, heat and fresh water #uxes through the Strait.
Speci"c patterns in the water-mass distributions are found, even for those water
masses that spread over the entire Strait. In the AW range, four modes are clearly
identi"ed based on their di!erent geographical distribution. In the DW range
the distribution patterns are closely related to topographic features. For instance, the
EBDW is con"ned to the Lena Trough and the Molloy Deep area. Even the
widespread NSDW shows preferential distribution patterns, with the warm, shallower
mode concentrated to the west and the cold, deeper mode to the east. A volumeric
analysis reveals that more than 65% of the strait is "lled with the two most abundant
water masses, the NSDW and the AW. By comparison, the volume occupied by all
water masses of polar origin does not exceed 20%. The remaining 15% of the total
volume is made by the two water masses formed in the Greenland Sea, the AIW and
the GSDW.
Two upper water masses, the AWF and the AWc, and one deep-water mass, the
NSDW, appear to be produced in Fram Strait, with production rates of about 0.2, 1.0
and 1.7 Sv, respectively, between 77.6 and 79.93N. The production of AWF, if not only
due to mixing with melt water, should be related to a consumption of PW, but the
latter is di$cult to identify in view of the large uncertainty on the PW transport
estimates. The required averaged loss of 0.8 Sv of AWw in the strait is consistent with
a participation of this water mass to the production of AWF. Still, the most part of this
loss contributes to local formation of AWc. A smaller source water mass for the AWc
may be the MAW, which would contribute 0.2 Sv through modi"cation at the
con#uence of the EGC with the RAC. The local production of NSDW is mainly
due to mixing of the NSDW supplied to the strait by the WSC with some UPDW,
CBDW and EBDW supplied by the EGC and the PC. The contribution from mixing
between the GSDW and EBDW should be much smaller, at least considering that,
on the average, no signi"cant GSDW de"cit is found in the strait. This de"cit is,
in any case, very di$cult to estimate as the GSDW is involved in a gyre-type
circulation.
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
1165
The net volume transport of PW through Fram Strait is southward and equal to
about 1 Sv at 78.93N. At the same latitude, only the net transport of AWw is
northward (0.2 Sv on an average) while, if all modes of AW are considered altogether,
the net volume transport of AW is southward and equal to about 1.7 Sv. The overall
net out#ow of about 5 Sv to the Greenland Sea through Fram Strait is therefore made
of a 2.6 Sv net transport in the upper and intermediate layer and a deep transport of
approximately the same magnitude. The deep transport is equally shared between the
EBDW, CBDW and UPDW, on one hand, and the NSDW, on the other. As
mentioned in SH1, the net out#ow combines a northward net #ow of 2 Sv in the WSC
system and a southward net #ow of 7 Sv in the EGC system.
The net fresh-water transport relative to 34.93 is southward, equal to about
2000 km yr\, and mainly associated with the PW #ow. The net heat transport
relative to !0.13C is northward. Due to transformation of the AWw, including
interaction with the atmosphere or with the ice, the magnitude of the net heat
transport through the strait rapidly decreases northward. Therefore, the contribution
of Fram Strait to the heat budget of the Arctic Ocean is rather achieved through ice
export. The convergence of the heat transports in the strait implies an area-averaged
surface heat loss of 50}100 W m\. The heat #ux involved in ice melting may be much
larger since melting by the ocean is mainly restricted to the vicinity of the ice edge.
Our analysis, when combined with the current distribution proposed in SH1,
reveals previously unknown details of the circulation in Fram Strait, in particular: (a)
a northern recirculation of the AWw from the WSC in the PC, along the lower slope
to the west of the Yermak Plateau, and its subsequent merging with the RAC and the
SFZC. Although the existence of such a recirculation was anticipated in SH1, the
similar water mass characteristics of the in#owing and recirculating branches revealed
by the present analysis demonstrate that such a recirculation indeed occurs to the
north of Fram Strait; (b) an anticyclonic circulation of the AWc east of the Molloy
Deep area, in agreement with the intermediate layer circulation proposed by SH1; (c)
an important southward recirculation of NSDWc from the WSC, not only in the EGC
via the SFZC, but also to the east along the lower Spitsbergen Slope. The recirculation
is almost complete and results in a southward net #ow of NSDWc to the Greenland
Sea, roughly equal to the local production in the strait; (d) a source of NSDWw to the
north of the Strait resulting in a net out#ow from the Arctic Ocean approximately
equal to the local formation rate of the water mass in the strait. Both modes of the
NSDW therefore out#ow to the Greenland Sea and only a small portion can be
identi"ed as an in#ow to the Arctic Ocean; (e) a cyclonic circulation of the EBDW in
the Molloy Deep area. A cyclonic #ow has already been noticed by SH1 in the bottom
#ow in this area, but has hardly been identi"ed in the 700 m-to-bottom depthaveraged circulation.
Acknowledgements
This work was part of the ESOP-1 project supported by the MAST II programme
of the Commission of the European Communities (contract n3 MAS2-CT93-0057).
1166
P. Schlichtholz, M.-N. Houssais / Deep-Sea Research II 46 (1999) 1137}1168
Appendix A. Interpolation of the hydrographic 5elds
Given a set of discrete data describing a scalar "eld (e.g., potential temperature or
salinity), '(x, y, z), we seek a smooth 3D interpolation function that is as close as
possible to these data. The problem reduces to minimizing the functional I:
I(')"(1!a)D(')#aR(')"min,
(A.1)
where D(') and R(') are the `dataa and `roughnessa norms, respectively, and
a(0)a)1) is a smoothing parameter. In our particular case where the estimate of
' is to be combined with an estimate of the velocity "eld obtained through a separate
interpolation (see SH1), the two estimates are made consistent by requiring that the
data norm in (A.1) include the same data as in SH1, namely N original data, '", and
"
G
N analysed data, '-, so that (see SH1 for details):
-
H
,"
,-
D(')"N\ w (' !'")#N\ w (' !'-),
"
G G
G
-
H H
H
G
G
(A.2)
where the ' and ' are the values of ' at the positions of the original and analysed
G
H
data, respectively, while the coe$cients w and w are local weighting factors having
G
H
the same dimension as ('")\ and ('-)\. The determination of the w 's, w 's and a is
G
H
G
H
explained in SH1.
The roughness norm:
R(')"
' \
' \
(
')# (D ') dx dy dz
F
F
¸
¸
(A.3)
is the sum of the integrals in the 3D space of the squares of the horizontal gradient,
', and laplacian, ', of '. These quantities are nondimensionalized using ' and
F
F
¸, the scaling lengths for the amplitude and the horizontal scale of the variations of ',
respectively.
To make the minimization problem (A.1) easily tractable requires reducing the
number of degrees of freedom of the problem. As in SH1, '(x, y, z) is expanded into
a set of M(M"9) hyperbolic}sine}logarithmic in the vertical and their horizontal
amplitudes are de"ned by polynomial functions discretized on a "nite element grid
with N nodes (N"1023). Requiring the "rst derivatives of the functional I with
respect to the expansion coe$cients to vanish, (A.1) is transformed into a system of
linear algebraic equations with N;M unknowns.
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